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Hello, I have a 2006 H200 with a 5.0l GXi motor.

When I put the boat in the water this year it started and ran perfectly, like it always does. This year I noticed that when I return the boat to normal cruising after running at part or full throttle the water temperature gauge drops from it's normal operating temperature reading to almost zero.

I am suspecting I need to replace the impeller but any suggestions would be welcome.

Thank you,

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Dan Robinson
2008 H200
5.0 GXi

First thing to do is to verify what your temp gauge is telling you. Get an IR temp gun and take readings under the thermostat housing (front of intake manifold) this will usually read about 10-15* lower than the dashboard temp gauge. Also take readings of both exhaust manifolds and exhaust elbows. It idle they will be about 100* but after running on plane they can go up to 135-150 or so. If your temp gauge reads all the way to the left that’s not zero it’s about 100*F. Normal temp ranges from 160-175. What can cause the problem you’re seeing is a stuck open thermostat when that happens even when you’re up on plane the engine may still run cool (130) at idle it can drop to 100*. All it takes is a small flake of rust or grain of sand. So take readings & see what you have…

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

+1 on the thermostat.

Your Volvo Penta uses an open system which draws water out of whatever you're sitting in to cool the engine, with an automotive-style thermostat to control the flow and maintain a (fairly) consistent water temperature. However, unlike an automobile with a closed system that uses a mixture of coolant containing rust-inhibitors, there's no such protection for the thermostat on your boat. After 20 years, if you've never replaced the thermostat, I'd bet money that it's stuck. Just be glad it's stuck open and not closed.

Because I am in salt water, I have this same problem every few years, when a cast iron engine rusts on the inside the iron flakes off and what happens is that it lodges in the 'stat and keeps it from closing all the way. Then it runs cool like that, 100* at idle and you have to run it on plane for it to get up to 150-160, it's all because the stat isn't closing all the way. Just to eliminate this nuisance, if I repower this boat I am adding a closed cooling system, which costs quite a bit of cash but is really worth it, because you don't have that problem at all, just like a vehicle your stat works perfectly for years and years, no internal corrosion, and much easier to winterize. Personally, I HATE raw water cooling on an inboard. Getting at those drain plugs every fall is a real trial. That's also why so many I/Os wind up with cracked blocks. The owners know they SHOULD squeeze themselves next to the engine to get the dang drain plugs out but they wait too long.
$1500 for a closed cooling system is well worth it. The engine runs at the temp GM intended; you have pressurized coolant which cools the hotter part of the cylinder heads better and also makes the boat worth more when you sell it.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II